It’s November, and salmon are currently leaving the oceans and returning to the rivers where they were born. During these epic waterfall-leaping, bear-dodging migrations, their bodies change. Their color darkens and reddens. The males develop hooked jaws, and sometimes humps. The red muscles that are so useful for long-distance swimming are replaced by fast-acting white muscles that can power sprints and jumps. And one of the most dramatic changes, and perhaps the least obvious one, happens in their eyes.
In rivers, flecks of mud and algae shift the underwater light away from the clear blue of the ocean and towards the red end of the spectrum. The salmon compensate for this: A simple biochemical switch in their retinas gradually enhances their ability to see infrared light. The salmon effectively transform their eyes into night-vision goggles, so they can see further into the murky water where they’ll fight, mate, spawn, and die.
This trick revolves around a pair of molecules that form the building blocks of all animal eyes: a protein called an opsin and a partner chemical called a chromophore. These two embrace each other tightly, forming a joint unit called a visual pigment. When light hits the pigment, the chromophore absorbs its energy and rapidly snaps into a different shape. Its contortions force its opsin partner to transform, too. The two transformations trigger a series of chemical reactions that end in an electrical signal traveling to the brain.